Removing entry trampoline and associated reversions

[Andy Lutomirski]

Hi all-

We had an unfortunate conflict.  Adrian did all the plumbing to get
entry_trampoline to play nicelyh with kcore and perf.  Meanwhile, I
was working on getting rid of the entry trampoline.  Adrian's code is
merged and mine is finally in good shape, and there's an obvious
conflict.

So I did a bunch of reverts, all but one of which were clean.  The
series is here:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/log/?h=x86/pti

and the messy revert is here: